Based on the timeline of marker colocalization with [3H]thy that we present here, we propose that a Pax6+, Chx10+ and syntaxin+ retinal progenitor cell population that is biased toward the amacrine/horizontal cell fate continues to proliferate through development in Chx10-Cre;RbLox/-; p107-/- retinas and eventually gives rise to retinoblastoma. This evidence concerns the gene RBL1 and retinoblastoma.